It has been well documented that APUD cells in various organs secrete polypeptide hormones which play a significant physiological role in the homostatsis of the organism. Although APUD cells have been found in the lung, no one has determined their specific physiological role because there are a relatively few APUD cells in the normal lung. We have quantitated the normal number, size and distribution and their response to injury in the hamster lung using fluorescent microscopy and histochemical techniques notably silver stains. An experimentally induced bronchial epithelium hyperplasia will be induced using nitrogen dioxide exposure and/or diethylnitrosamine and by nerve growth factor in an effort to increase the cell population. The spatial relationship of APUD cells and globlet cells in the tracheal epithelium will be analyzed since it has been suggested that processes from the Kulschitsky cell (K cell) are in direct contact with goblet cells or that APUD cells may be transformed into goblet cells. This close anatomic relationship of these cell types suggests the hypothesis that APUD cell secretions (polypeptides) may regulate or influence mucus secretion of goblet cells in the respiratory mucosa. Studies are in progress to characterize the polypeptides within the APUD cells of unstimulated cell populations as well as those stimulated by agents causing bronchial epithelial hyperplasia.